The Germans captured millions of Soviet troops in the early part of the war, and Soviet weapons were widely used. Not only SMG's but also Tokarev pistols were converted to use 9mm (08) ammunition. Rifles like the Tokarev SVT-40 were widely used and Russian ammo was captured in such huge quantities that it was actually put into the German supply system.
(It is worth noting that the Germans considered Russians "untermenschen" and treated Soviet war prisoners in the most brutal manner, executing hundreds of thousands, and letting hundreds of thousands more die of disease and starvation in prison camps. This is one of the reasons the Red Army was not especially nice to Germans when they in turn invaded the German homeland, and why the Russians refused to let German POW's return to Germany immediately after the war.)
Jim